GROWTH

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GROWTH

• The Onion Root Tip As a Model for Growth

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Click on the red spots to find out what is
happening in each zone.
ZONE D
ZONE C
ZONE B
ZONE A
l.s. root tip
Root Cap
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Zone of Cell Division
Information Note New cells are CUBOIDAL in
shape Have thin cellulose cell walls Have dense
cytoplasm with no vacuole
Information New cells are produced by MITOSIS
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Zone of Expansion
Information Cells in this region have Thin
cellulose cell walls (primary) Small
vacuoles cytoplasm with low ? (water potential)
Information ? Of surrounding solution is HIGH
so water moves into vacuole of cell
As a consequence of water movement the volume of
the cells increases and pushes out the thin cell
walls
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Zone of elongation
Information The arrangement of cellulose fibres
in the cell wall results in the expansion of
cells only in a lengthways direction
Information Cells in this region are typified
by Large vacuoles Column shape Thin (primary)cell
walls
Water continues to enter the cells down a water
potential gradient as a consequence the cells
elongate
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Zone of Differentiation
As the cells mature, it lays down more cellulose
inside to form a secondary cell wall. This
prevents further growth but allows for
DIFFERENTIATION e.g.
The end cell walls may become perforated as the
cells develop into PHLOEM SIEVE TUBE ELEMENTS
If the secondary cell wall contains LIGNIN, then
the cells will develop into XYLEM or SCLERENCHYMA
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A simple experiment A bean root can be marked
at 1mm intervals. The bean can be left to grow
and the root observed every 12 hours.
Note the region of greatest elongation is just
behind the tip